Boosting High-Voltage Practical Lithium Metal Batteries with Tailored Additives

Highlights FGN-182 electrolytes exhibit highly reversible Li plating/stripping with an average Coulombic efficiency reaching up to 99.56% determined from Auerbach’s test. The gas-evolution process of LiNO3 in high-voltage lithium cobalt oxide (LCO) cathodes is revealed by in situ differential electrochemical mass spectrometry. Pouch cells equipped with high-loading LCO (3 mAh cm−2) cathodes, ultrathin Li chips (25 μm), and lean electrolytes (5 g Ah−1) using optimized electrolyte (FGN-182 + 1%HTCN) demonstrate outstanding cycling performance. Supplementary Information The online version contains supplementary material available at 10.1007/s40820-024-01479-1.


S1.3 Preparation of LiNi1/3Co1/3Mn1/3O2 (NCM111) and LiCoO2 (LCO) cathodes
For the coin cells, the NCM111 powder, carbon black, PVDF with a weight ratio of 90:4:6 was mixed and stirred for 12 h to form homogeneous slurry.The slurry was further casted on Al foils and following dried in vacuum at 80 °C for 12 h and roll-in treatment to acquire NCM111 cathode electrodes.For the pouch cells, the dual-layer coated LCO cathodes was provided by Contemporary Amperex Technology Limited.

S1.4 Electrochemical tests
The electrochemical performance of the electrodes was tested in coin cells (CR2025) which were assembled in an Ar-filled glove box (O2 < 0.01 ppm, H2O < 0.01 ppm).The separator here was polypropylene film (Celgard 2400).The conventional Coulombic efficiency (CE) of Li was measured in Li||Cu cells.Before the test, the Li||Cu cells were first discharged at a current density of 0.5 mA cm -2 for 8 h and charged to 1.2 V to strip all of the plating Li to stabilize the initial SEI film.During the tests, a controllable amount of Li was plated on the Cu foils at a constant current density, and a fixed voltage was carried out to strip the Li from the Cu foils.The CE of Li is defined as the ratio of the amount of Li during the stripping process and plating process.The innovative CE of Li was also measured in Li||Cu cells according to Auerbach's test.The Li||Cu cells were activated by the as-mentioned method to stabilize the initial SEI film, and plating a certain amount of Li (depending on the excess amount of Li) on Cu foils to form Li||Cu@Li cells, then charging/discharging at 0.5 mA cm -2 for 2 h for 10 cycles, stripping all of the plating Li to calculate the CE, which is defined as: Where A is the first plating amount of Li on Cu foils, B is the last stripping amount of Li from Cu foils, and C is the amount of charging/discharging process.
The NCM111||Li coin cells were first activated at each lower current density for 2 cycles with a voltage range from 3.0 to 4.2 V and then tested the long-term cycling performance at higher current density.The ratio of negative/positive (N/P) of the cells was 2.06.Similarly, the LCO||Li pouch cells were also activated before long-term cycling.The ratio of N/P is decreased from 2.0 to 1.7 due to the charging cutoff voltage (increase from 4.2 to 4.4 V).The voltage windows for LCO||Li pouch cells were from 3.0 to the different cutoff voltage mentioned.In addition, the amount of electrolyte is fixed at 5 g Ah -1 .
The electrochemical impedance spectra (EIS) and linear sweep voltammetry (LSV) experiments of the cells were tested on a CHI660E (Chenghua) electrochemical analytical instrument.The EIS operating frequency ranged from 10 -2 to 10 5 Hz, and the LSV profiles were obtained in the voltage range of open-circuit to 5.5 V at a scan rate of 5 mV s −1 .The Li + transference number (tLi+) was determined using chronoamperometry (applying a DC voltage of 10 mV) and EIS (with frequencies ranging from 10 -2 to 10 5 Hz) in Li||Li symmetric cells with various electrolytes.The tLi+ was obtained according to the Bruce-Vincent equation [S1]: where Δ denotes the voltage polarization applied.I0 and Iss are the initial and steadystate currents of the cell.R0 and Rss are the initial and steady-state resistances, respectively.

S1.5 Characterizations
A field emission scanning electron microscope (FESEM, Hitachi S-4800) was used to observe the morphologies of plating Li on Cu foil and Li anodes before and after cycles.
The sample was transferred from the argon-filled atmosphere to the SEM vacuum chamber in an instant.Transmission electron microscopy (TEM, JEOL JEM-2100) was conducted to analyze the detailed structural information of materials.X-ray photoelectron spectroscopy (XPS, PHI 5000 VB III) was carried out to investigate the surface information of the materials, and the binding energies reported herein were corrected with reference to the C-C/C-H signal at 284.8 eV.An Al Kα monochromatized radiation (hν=1486.6eV) was employed as an X-ray source.Atomic force microscopy (AFM) tests were obtained by SPM 5500 (Put in an Ar-filled glove box).The gas evolution of the samples was monitored using in situ differential electrochemical mass spectrometry (DEMS) (Linglu Instruments, Shanghai, i-DEMS QMG220).

S1.6 Calculation methods
The density functional theory (DFT) calculations were conducted utilizing the PW and PP modules incorporated in the Quantum ESPRESSO distribution [S2].Interactions among nuclei and core electrons were described employing the ultrasoft method within the framework of the generalized gradient approximation (GGA) using the Perdew-Burke-Ernzerhof (PBE) functional.A kinetic energy cutoff of 30 Ry with a chargedensity cutoff of 300 Ry was employed to ensure energy convergence.Fermi-surface effects were addressed using the Methfessel-Paxton smearing technique with a smearing parameter of 0.02 Ry.Prior to calculation, models of DME, DEGDME, LiFSI, and LiNO3 were relaxed to optimize their structures.A LiCoO2 slab model with four layers was chose to simulate the cathode LiCoO2.Both lower layers were fixed to simulate the phase of LiCoO2, others were relaxed to simulate the surface.The vacuum layer of 15 Å was added in z axis to avoid the influence from periodic slabs.K-point sampling in the Brillouin zones was conducted using the gamma point.Van der Waals corrections were incorporated using the DFT-D3 method.The molecular dynamics simulations were performed using the GROMACS 2023.02software package [S3], with all molecules described using the Oplsaa force field [S4].The initial configurations of all simulation systems were obtained by uniformly mixing the components of the system using the Packmol software package [S5].Firstly, energy minimization was conducted for 3000 steps using the steepest descent method to eliminate unreasonable atomic overlaps.Subsequently, relaxation was performed for 100 ps at 298.15 K under both NVT and NPT ensembles, with an integration time step of 1.0 fs.Finally, production simulations were conducted for 50 ns under the NPT ensemble.In the MD simulations, periodic boundary conditions were applied in all directions, with a time step of 2.0 fs.The temperature was maintained at 298.15 K using the V-rescale thermostat with a coupling time of 0.5 ps, and the pressure was controlled at 1 bar using the Parrinello-Rahman barostat with a coupling time of 2.0 ps.All hydrogen-containing bonds (C-H, O-H) were constrained using the LINCS algorithm.The long-range electrostatic interactions were treated with the particle mesh Ewald (PME) method, with a cutoff of 1.2 nm applied to calculate short-range electrostatic and van der Waals interactions.Trajectory analysis was performed using the GROMACS software package.

Fig. S18
Fig. S18 The in-depth XPS spectra of SEI layer formed on Li metal cycled in FGN-182 and FGN-180 electrolytes of O 1s spectra

Table S1
Comparison of half-cell CE with reported electrolyte optimizations in the literature

Table S2
Number and density of molecules in the simulation box

Table S4
Comparison of full-cell cycling performance with reported electrolyte optimizations in the literature